Where did the speed of light come from and why is it so stubborn?
NASA, ESA, CXC, SSC
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If you’ve taken a physics class, you likely have “memorable” instances of measuring light speed, spending hours setting mirrors, lenses, and light sources just right to achieve the result: just under 300 million meters per second. This figure is a fundamental constant in physics and vital for comprehending the universe.
When observing space, light is our primary resource. While we have other means, like gravitational waves, they currently offer limited insights, so I might be exaggerating a tad. Almost all advancements in astronomy and cosmology derive from collecting light that has traversed from the edge of reality over millions, or even billions, of years. Light from our nearest star takes over four years to reach us. The duration it takes for light to travel may be one of the most practical yet least intuitive aspects of physics.
Humans have debated light’s speed long before we truly understood light itself. For centuries, many intellectuals believed that the glowing in certain animals’ eyes at particular angles indicated they emitted light, resembling a lantern. Nonetheless, they debated whether light traveled instantaneously or required time to propagate, a question not thoroughly tested until the 17th century.
An early endeavor to quantify it involved placing a lantern at a distance and measuring the time difference between it lighting up and the observer seeing the light. This method proved ineffective (Galileo and his peers failed to attain conclusive measurements because the lantern was too close), leading scientists to explore more complex and accurate approaches. The first effective instrument was developed in 1675 by Ole Römer while measuring Jupiter’s moon Io’s orbital period. He observed that the period seemed to vary as the distance from Earth to Jupiter fluctuated, which seemed perplexing. Why would Io’s orbit correlate with Earth’s positioning? The only variation was the time it took for light to travel from Io to Earth, diminishing as the two grew closer. A colleague, Christian Huygens, calculated that light’s speed was around 220,000,000 meters per second. Although this estimate lacked precision due to unknown earthly movements, it established a foundation for later refinements. By the early 18th century, measurements were within a few percent of the current consensus of light’s speed in vacuum: 299,792,458 meters per second.
This prompts two inquiries: Why is the speed of light seemingly arbitrary, and why is there a speed limit at all? The first question is straightforward, linked to our units. Meters and seconds (or miles and hours) originated from human experiences. For instance, a mile equals 1,000 steps and has no relation to fundamental constants. The second question is more complex, entwined with special relativity.
The answer lies in perhaps the most recognizable equation: e=mc2. This equation implies that energy and mass can be interchanged. When objects move at extremely high or relativistic speeds, I like to think of them possessing momentum, blending mass and velocity. To increase an object’s speed, we must continually supply more energy. A massive object achieving light speed would require infinite momentum, equating to infinite energy or mass. This situation is unattainable. As an object nears light speed, its mass escalates, making further acceleration unfeasible. Light, having no mass, circumvents this dilemma.
Moreover, special relativity illustrates that an outside, stationary observer would perceive something quite unusual. When an object travels at relativistic speeds, time appears to slow down from an external viewpoint. If I were moving away from you at 99% of light speed, I’d observe my aging decelerating. This phenomenon is termed time dilation. Concurrently, another effect, length contraction, would have you notice that I’m shrinking increasingly as I accelerate. From my frame of reference, I wouldn’t perceive time slowing down or my stature diminishing, but from your outlook, the closer I get to light speed, the shorter and more ageless I appear.
Herein lies a paradox: if I somehow reached light speed, time would seemingly stop for an outside observer as my height approaches zero. I would cease to exist, along with time and space. Luckily, the laws of physics preclude that scenario. Only massless entities can attain that speed limit: photons, gluons, and gravitational effects. Nothing surpasses light speed through space and time.
Rather than feeling disheartened by the universe’s speed limitations, we should celebrate them. The speed of light carries a crucial consequence: it underpins the whole notion of causality. All physics, and our comprehensive understanding of everything, hinges on the principle that effects always follow causes, never the other way around.
Consider this: as I approach light speed, you observe my time slowing down. It will cease entirely when I attain light speed. Should I exceed light speed, from your perspective, I’d be reversing time. If I transmitted a signal faster than light, a hypothetical message defying physics, you’d receive it before I sent it. Absent a universal speed limit, discerning which events caused which effects would be impossible, rendering the universe largely incomprehensible.
Finally, here’s a thought-provoking notion: if all signals require time to travel, and time progresses variably in frames of reference moving at different speeds, what does simultaneous meaning? If I wink at my reflection, the reflected wink arrives slightly later than my physical action, due to light needing to bounce off my face, towards the mirror, and back into my eyes. If two events simultaneously occurred across the universe, I must ask, “By whose standard?” Depending on the distance separating two locations, event 1 might have occurred first for one observer, while event 2 happened prior to event 1 for another. There is no objective simultaneity, no definitive “same time.” This reality stems solely from light’s finite speed. Fascinating, right?
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Source: www.newscientist.com
